Windows Help files not supported in Vista
For all the effort they put into building operating systems that are backwards-compatible, they then go and choose not to include support for Help files that are built to use the old WinHlp32.exe program. This probably will not affect a huge number of Vista users, but the ones that it does affect are two important demographics, in my opinion. The people that are most likely to be relying on a program that still uses that old style help file are power users, and the not-so-savvy "nervous Nelly" users.
For example: a power user might have some utility that they absolutely can't live without. In Vista, they may find that the utility still works, but they can no longer view the contextual help for that program. This class of user can probably fend for themselves though; they can still go and download WinHlp32.exe from Microsoft and get access to those old help files.
But what about the proverbial elderly person who is happy with some ancient version of some program, and isn't willing to switch to a new or different one? When they buy that new computer, they will be completely lost when they can't get the help they need.
Okay, maybe it's not so big as all that, but it's definitely a pain in the ass. So there.
[Via Chris Pirillo]